1. Field of the Invention
The present invention relates to a control device for a die cushion mechanism, in particular, to a control device capable of controlling force generated by a die cushion mechanism of a press machine.
2. Description of the Related Art
It is known that a press machine, for bending, drawing or punching, etc., is provided with a die cushion mechanism, as an attached device, for applying a predetermined amount of force or pressure, to a movable support member (generally called a slide) supporting a first mold for press working, from another support member (generally called a bolster) supporting a second mold. The die cushion mechanism is generally configured such that the slide (or the first mold), moving in a mold-clamping direction, directly or indirectly collides with a movable element (generally known as a cushion pad) held at a predetermined pressure, and until the molding is finished, the cushion pad is moved with the slide while applying force or pressure to the slide. During this operation, it is possible to prevent wrinkles from forming in a workpiece to be pressed by, for example, the clamping an area around a site of the workpiece to be pressed between the cushion pad and the slide.
Many conventional die cushion mechanisms use hydraulic or pneumatic units as driving sources. However, control by a hydraulic or a pneumatic unit can only be carried out under constant pressure. It is preferable that the pressure during drawing be varied in response to the amount of the drawing; however, the amount of pressure cannot be varied in the hydraulic or pneumatic unit.
In recent years, a die cushion mechanism using a servomotor as a driving source has been used to carry out force control, with a fast response, as described in Japanese Unexamined Patent Publication (Kokai) No. 10-202327. In the die cushion mechanism described in this publication, a cushion pad positioned below a slide of a press machine may be upwardly and downwardly moved by a servomotor, corresponding to the rise and fall of the slide. The servomotor operates by force control based on a force command value predetermined corresponding to the position of the cushion pad, and adjusts a force or pressure applied to the slide from the cushion pad, while moving the cushion pad with the slide. Collision and the pressure, between the slide and the cushion pad, are detected by detecting a load applied to an output axis of the servomotor via the cushion pad.
In a die cushion mechanism having a servomotor as a driving source (hereinafter, referred to as a servo die cushion), the force control is carried out by a force control loop including P control or PI control using a force command value and a force detected value. However, since the die cushion controls the force while moving with the slide, it is difficult to control the variability of the force due to the relative movement of the die cushion to the slide, by means of only the force control loop, the response of which cannot be considerably raised. In order to solve the problem, for example, Japanese Unexamined Patent Publication (Kokai) No. 2006-130524 discloses a technique to control the variability of the force due to the relative movement of the die cushion to the slide, by correcting a speed command value generated by the force control loop using a speed detected value of the slide. Due to this, the workload of the force control loop may be reduced and the response to a force command may be improved.
Generally, when the servomotor as the driving source has a large amount of torque, the preferable force control may be carried out. However, when the torque of the servomotor is not sufficient, such as when the number of press actions per unit time is large relative to the torque of the servomotor, a force detected value may not be properly controlled corresponding to a force command value. This is because a conventional press machine is relatively large and the rigidity of the slide and the die cushion cannot be sufficiently increased. In such a case, a gain of the force control loop cannot be sufficiently increased, and thus a response of the force control loop cannot be improved.
Concretely, as shown in FIG. 4a, when the servomotor has a sufficient torque, the force detected value (a dashed line) smoothly comes closer to the force command value (a solid line). However, as shown in FIG. 4b, when the torque of the servomotor is not sufficient, a force deviation may be increased due to an overshoot in which the force detected value (a dashed line) substantially exceeds the force command value. In such a case, an overshoot and an undershoot are repeated until the force deviation converges, resulting in much time being required until the force deviation converges.